1. Field of the Invention
The present invention relates to a light-emitting device that has a light-emitting element in each pixel.
2. Description of the Related Art
A light-emitting element itself emits light with high visibility, needs no backlight that is required for a liquid crystal display device (LCD) to be suitable for reduction in thickness, and has no limit of viewing angle. Therefore, a light-emitting device using a light-emitting element has been attracting attention as an alternative display device to a CRT or an LCD, and has been putting into practical use. An OLED (Organic Light Emitting Diode) that is one of light-emitting elements has a layer including an electroluminescent material from which luminescence (electroluminescence) can be obtained by applying an electric field (hereinafter, referred to as an electroluminescent layer), an anode, and a cathode. By combining a hole injected from the anode and an electron injected from the cathode in the electroluminescent layer, luminescence can be obtained. The luminescence that can be obtained from the electroluminescent layer includes luminescence (fluorescence) on returning to the ground state from a singlet excited state and luminescence (phosphorescence) on returning to the ground state from a triplet excited state.
The obtained light can be extracted from any of the anode side and the cathode side in principle. In the case of an active matrix light-emitting device, it is preferable that the light is extracted from the electrode, the anode or the cathode on the side that is farther away from a substrate over which a wiring or a gate electrode of a transistor is formed since a high extraction efficiency can be kept independently of a decrease in aperture ratio with high resolution. The light can be extracted from the electrode by using either method of forming the electrode thinly enough to transmit light or forming the electrode with the use of a transparent conductive film. However, in the former method of the above-mentioned two methods, it is difficult to enhance a light extraction efficiency sufficiently since transmissivity has a limit.
On the other hand, in the case of using the latter method, it is relatively easy to enhance a light extraction efficiency as compared the former.
However, when sputtering is used to form a transparent conductive film typified by ITO (Indium Tin Oxide) on an electroluminescent layer, there is a problem that a layer including an organic material is subjected to damage (sputter damage) in the electroluminescent layer. In the case of using evaporation to form a transparent conductive film, damage to the layer including the organic material can be suppressed. However, in this case, an electrode to be formed is reduced in transmissivity and is increased in resistivity, which is undesirable. Now therefore, it is desired to propose a light-emitting device, where sputtering is used to form an electrode on an electroluminescent layer without damage to a layer including an organic material.
In order to perform displaying in full color with a light-emitting device, a method of using three kinds of light-emitting elements corresponding to R (red), G (green), and B (blue) or a method of combining a light-emitting element that emits white light and a color filter is generally used. However, in the case of the former method, it is necessary to enhance the color purity of luminescence corresponding to each of R (red), G (green), and B (blue), and then spend a lot of cost and time to optimize an electroluminescent material and a device structure. In the case of the latter method, light shielded by the color filter is wasted, and then, there is a problem that no high luminance can be obtained for the power consumption.